The present invention relates to a method and an apparatus for detecting and characterizing an event, such as a leak, which causes a pressure transient to occur in a pipeline and, more particularly, to a method and an apparatus for making use of the amplitude, time of arrival, and direction of travel of a wave front caused by the pressure transient to locate the origin of the wave front, determine the size of the leak, and derive its time of occurrence.
It has long been known that most pipeline operational events such as operation of valves, start-up or shut-down of pumps, or a break in the pipeline can result in a sudden pressure change at the location of the event. This pressure transient immediately forms a pressure wave which travels both up-stream and down-stream from the site of the event at the speed of sound in the fluid flowing in the pipeline.
Because of the effect of a leak on the environment, it is imperative to identify the location of the leak quickly, and to determine its magnitude. The nature and urgency of the required remedial action can then be determined.
Various techniques are known for determining such information from the wave front. For example, U.K. Patent Specification No. 1,438,237 determines the origin of a leak from the difference in time of arrival of a wave front at detectors on either side of the leak. Published European Application No. 0 042 212 A1 obtains the same information from the amplitude of the wave front at locations up-stream and down-stream from the leak. U.S. Pat. No. 3,851,521 discloses a technique which can locate the position of a leak based on a difference in arrival time of the wave front at detecting stations on either side of the leak even when the spacing between pairs of detecting stations along the pipeline is unequal.
Once the location and magnitude of the source of the wave front are determined, this information can be used to decide whether it represents a leak, operation of valves or other equipment in the pipeline, or is due to noise.
Various sources of inaccuracies can significantly affect the determinations made by the prior art approaches. For example, uncertainty exists as regards the degree of precision of a time standard to which arrival times at different detecting stations are referenced. Also, the accuracy with which the travel velocity of the wave front in the pipeline is ascertained has some uncertainty, and particularly as it changes with different pipeline fluid constituents. The prior art is incapable of suitably dealing with these sources of inaccuracies.
Results obtained with the prior art are vulnerable to errors due to a plurality of events occurring substantially simultaneously. For example, in a protected 100 km section of the pipeline (monitored by stations spaced along the pipeline) and for a wave front velocity of 1100 m/sec, a wave front takes 90 seconds to travel this distance. If a valve on this same section of pipeline is operated within, say, 90 seconds or so after a leak has sprung, both events will cause a wave front to arrive at the monitoring stations and may be interpreted as corresponding to a single event. Of course, a pressure transient event can also occur outside of the protected section of the pipeline, and a pass-through wave front emanating therefrom will traverse the protected section. This class of signals must also be suitably identified and properly dealt with. Echoes or reflections can also be a serious concern. It is important to be able to separate the wave front signals arriving at various monitoring stations along the pipeline and which are caused by a leak, for example, occurring in the protected section from signals due to normal pipeline-operation-related events occurring outside the protected zone, and from reflections or noise.
As pipeline constituents are changed, various parameters required for locating the source of the event also change. It is important to be able to readily and accurately derive updated values for such parameters, but the prior art is incapable of doing so.